1998 Volvo V70 sevice light is on

For late versian of the instrument cluster.
To reset the service reminder indicator (SRI) carry out the procedure as following:

Ignition off
Press and hold in the trip odometer reset button
Switch on ignition
Continue pressing in the reset button for 10 seconds until Service Reminder Indicator starts flashing.
Release reset button within 4 seconds
If that does not work then you have the early style cluster and the only way to reset it is with a scan tool.

If you have the Yazaki instrument panel then there is a button you can push in using something thin like a paper clip.